Dewatering in a paper machine is normally made in three different steps, and the earlier in the machine the dewatering is made, the more cost-effective it is:
1. The forming section removes water by drainage from a web by means of suction boxes, by vacuum, table rolls, foils, etc. In older machines of the Fourdrinier type, such dewatering takes place on one side through the so-called extended wire. In modern wire sections, it has been possible to increase the dewatering by injecting the stock between a pair of wires, which permits the water to be drained from both the upper and the lower side. This entails that the wire section can generally be made shorter and more compact. The web is here still weak and is normally passed to the press via a pick-up felt. The purpose of the latter development was to improve the paper characteristics, inter alia, by making it possible to reduce the twosidedness of the web. This is also a condition for running in a stable way at high speeds. PA1 2. In the press section, the web is drained by being subjected to pressure in press nips between one or, alternatively, two felts. The felt covering removes the water and is reconditioned. To make it possible to increase the efficiency in this section, the number of press nips have in many cases been increased to four. Another solution for increasing the capacity has been to replace traditional press nips with so-called extended nips, where the pressing takes place with a shoe as support. In this type of presses, use is made of flexible belts which constitute the roll coating around the shoe. The demands placed on this type of polymer-coated process belts are that they should have a smooth surface and yield a uniform distribution of pressure when passing over the shoe. After the press section, the paper web has such a strength and dry solids content that it may be exposed to a certain amount of tension in the next transfer to the dryer section. In the future, machine concepts will be available, where the web is supported via various belts, etc., through the entire machine. PA1 3. The dryer section dewaters by the web being pressed against steam-heated drying cylinders. There are a number of different solutions to the arrangement of these cylinders and the orientation thereof in order to increase the efficiency in the dryer section.
In wire sections of the twin-wire type, as stated under 1 above, it is possible to have new options regarding the design of suitable fabrics. This depends on the facts that both fabrics work supported by different machine elements in the wire loop, and that these are placed close to each other. The fabric is subjected to a more constant load around the wire loop than in the older wire sections of the Fourdrinier type. In these, a very high stability was required in, e.g., the machine direction for the fabric to resist the pulsating strains that may occur around the revolution.
Known fabrics exist in both single-layered and multilayered design. These include one or more thread systems in the machine direction (MD) and the cross direction (CD), respectively. In order to achieve wear resistance, it is common to choose coarse threads in the CD in the fabric bottom, which is turned to the supporting parts of the machine. Regarding these reinforcements in known fabrics, the reinforcing threads are generally coarse and have higher wear resistance than other threads. It is common to use PA in the bottom. This material does not have higher modulus than PET.
Older single-layered fabrics of metal (bronze alloys) had the disadvantage that their running time was too short. In the 1970's, fabrics made of polymeric material made a breakthrough. Single-layered fabrics with 2-shed surface were, however, not stable enough, so multi-shed patterns (5-shed and above) were run with some success. These single-layered fabrics had too low a stability and too short a running time. To a large extent, they were replaced by multilayered designs of the type double-layered and triple-layered fabrics. Single-layered fabrics have almost completely disappeared from the market.
The drawbacks of today's multilayered fabrics are, among other things, that they do not cope with the high machine speeds that are desired and that they drag too much water. Water currents and pulsations may occur in the fabric. Prior-art single-layered fabrics with a 2-shed pattern especially suffer from the drawback that they are unstable due to a high degree of waviness (crimp) in the individual threads.
The object of the present invention is to provide a thin and stable fabric that especially functions in a twin-wire section and that produces good dewatering also at high machine speeds, above 2000 m/min. The fabric should be easy to keep clean and should drag a minimum amount of water. This results in a better production economy while retaining the paper quality. If the fabric is to be used in the dryer section, a minimum of air friction and thin boundary layers around the fabric are desirable.